Challenge 3: A Type Alias for a Callback — Solution

typealias ValidationResult = Pair<Boolean, String>
typealias Validator = (String) -> ValidationResult

fun validateUsername(): Validator = { username ->
    if (username.length >= 3) {
        true to "Valid username"
    } else {
        false to "Username must be at least 3 characters"
    }
}

fun main() {
    val validator = validateUsername()

    val result1 = validator("ab")
    val result2 = validator("alice")

    println(result1)
    println(result2)
}

/*
Output:
(false, Username must be at least 3 characters)
(true, Valid username)

Notes:
- ValidationResult is just a more readable name for Pair<Boolean,
  String> — it's fully interchangeable with that Pair type, not a
  distinct type, so "true to \"Valid username\"" (which produces a
  Pair<Boolean, String>) can be returned directly where a
  ValidationResult is expected.
- Validator is a readable name for the function type (String) ->
  ValidationResult — without the alias, validateUsername()'s return type
  and the validator variable's type would both have to spell out
  "(String) -> Pair<Boolean, String>" in full, which reads far less
  clearly.
- validateUsername() returns a lambda matching the Validator shape, and
  that lambda is then called twice with different usernames in main().
*/